1. Field of the Invention
This invention relates to printing materials, and devices incorporating such materials, for indicating changes in temperature, and particularly for indicating whether a rise in temperature above a predetermined value has taken place.
Temperature-sensitive devices are required, for example, for monitoring the storage of commodities such as drugs, food-stuffs, and chemicals. In such an application the device is typically attached to the stored commodity, and is required to indicate whether the. temperature of the commodity has risen above a predetermined temperature at any time since the device was attached to the commodity.
In our copending British Patent Application No: 8526680 there is disclosed a device for indicating a rise in temperature above a predetermined value, comprising a quantity of a material which changes from a solid state to a liquid state at the predetermined temperature value, the material being contained in a fracturable enclosure. In use, the device is attached to the commodity the temperature of which must be maintained below the predetermined temperature value, and is activated by the fracturing of the enclosure by pressure applied from outside the enclosure after the device has reached a temperature below the predetermined temperature value. Any subsequent increase in temperature of the device above the predetermined temperature value will cause the material to become liquid, so that it flows out of the previously-fractured enclosure, and causes a detectable change in the device which is not reversed by subsequent reduction of the temperature of the device below the predetermined value.
Such a device, and method of using the device, suffers from the disadvantage, however, that the enclosure must be fractured after the device has reached a temperature below the predetermined temperature. Where, in a typical application, the device is attached to such a commodity as frozen meat for sale in a supermarket, in a typical packaging process the meat prior to freezing is packed in a suitable wrapping material, and labelled with a label containing information such as the weight, price and sell-by date of the meat. It would be particularly convenient if a material for indicating a rise in temperature above a predetermined value at which the frozen meat would spoil could be incorporated into such a label, and if the material were activated as the label is dispensed. It is clear that a device as described in British Patent Application No: 8526680 could not be used in this way. Furthermore, the problem also arises that there is typically a delay of up to two hours before the packaged and labelled meat is placed in a deep freeze in order to reduce its temperature.